Recently, advances have been made which strike a more desirable balance between maintaining fuel efficiency and reducing the percentage of particulate emissions in fuels through the use of blends of petroleum based fuel with alkyl esters of the fatty acids contained in vegetable oils or animal fats. These alkyl esters are commonly referenced to as “biodiesel”. Substantially pure alkyl esters, such as methyl or ethyl esters of fatty acids, are generally preferred in biodiesel over the use of the vegetable oils and animal fats themselves because the alkyl esters have a viscosity that is more appropriate to diesel fuel. Through the use of these fuel blends, researchers have attained reductions in particulate emissions from diesel engines. See, e.g., M. Bender, Bioresource Technol., 70, 81 (1999); M. Diasakou et al., Fuel, 77, 1297 (1998); T. Ogoshi et al., J. Am. Oil Chem. Soc., 62, 331 (1985).
The production of biodiesel has received also extensive interest as a result of this fuel's desirable renewable, biodegradable, and nontoxic properties. See, e.g., G. J. Suppes et al., J. Am. Oil Chem. Soc., 78, 839 (2001); G. Kildrian et al., op. cit., 73, 225 (1996); J. Encinar et al., Ind. Eng. Chem. Res., 38, 2927 (1999). These fatty acid alkyl esters can be prepared by the transesterification of triglycerides in vegetable oils with short-chain alcohols (e.g., methanol and ethanol) using homogeneous alkali catalysts such as alkoxides. For example, soy diesel (methyl soyate) is made commercially by an energy and labor-intensive process wherein soybean oil is reacted with methanol at 140–150° F. (sometimes under pressure) in the presence of sodium methoxide. Isolation of the desired methyl soyate from the highly caustic (toxic) catalyst and other products such as glycerol, involves a precise neutralization process with strong acids, such as hydrochloric acid (HCl), and extensive washes with water to remove the resulting sodium chloride (NaCl) salt. Also, because of glycerol's high boiling point, it must be separated from the sodium chloride salt by vacuum distillation in an energy intensive operation. As more alkyl soyates with different alkyl functional groups, such as ethyl and isopropyl soyates, are being rapidly developed to meet the growing needs of various applications, the level of difficulty in separating the corresponding catalysts, e.g., sodium ethoxide and sodium isoproxide catalysts, respectively, will unavoidably escalate due to the increasing solubility of these basic catalysts in the reaction mixture. Therefore, biodiesel is currently not cost competitive with conventional diesel fuel.
To improve the economic outlook of biodiesel and alkyl esters in general, the feedstock selection becomes critical. In particular, oil feeds containing high free fatty acid content, such as found in beef tallow or yellow grease, are significantly less expensive than vegetable oils, such as soybean or rapeseed oil (F. Ma et al., op. cit., 37, 3768 (1998). These high free fatty acid feedstocks present significant processing problems in standard biodiesel manufacture since the free fatty acid is saponified by the homogeneous alkali catalyst that is used to transesterify triglycerides leading to a loss of catalyst as well as increased purification costs (D. G. B. Boocock et al., J. Am. Oil Chem. Soc., 75, 1167 (1998).
One approach for improving the processing of high free fatty acid oils is to first esterify the free fatty acids to alkyl esters in the presence of an acidic catalyst such as a mineral acid. The pretreated oils in which the free fatty acid content is lowered to no more than 0.5 wt % can then be processed under standard transesterification reaction conditions (H. N. Basu et al. (U.S. Pat. No. 5,525,126)). This pretreatment step has been successfully demonstrated using sulfuric acid (S. Koona et al., European Pat. No. 566047 (1993)). Unfortunately, use of the homogeneous sulfuric acid catalyst adds neutralization and separation steps as well as the esterification reaction to the overall process.
Surfactant-templated mesostructured materials have received a great deal of attention as potential catalysts, sensors and adsorption agents owing to their combination of extremely high surface areas and ordered, flexible pore sizes. For example, mesoporous sieves of the type MCM-41 are prepared by thermal treatment of silaceous gels formed by the polymerization of alkoxysilanes around surfactant micelle templates in aqueous base, followed by removal of the surfactant to yield a matrix comprising fine pores in a cylindrical array. The physical and chemical properties of these mesoporous materials can be modified by incorporating functionalized organic groups, either by grafting on the preformed mesopore surface or by co-condensation using functionalized substituted trialkoxy silanes during synthesis. See, e.g., D. Zhao et al., Science, 279, 548 (1998); A. Stein et al., Adv. Materials, 12, 1403 (2000); and W. M. Van Rhijn et al., Chem. Commun., 317 (1995). Organic-inorganic hybrid mesoporous silicas formed by co-condensation with thio-containing silanes, followed by oxidation of the SH groups yield pores functionalized with sulfonic acid groups. The direct co-condensation synthesis technique in which the mesostructure and functional group are simultaneously introduced, appears to be a desirable route for incorporating functional groups because it has been shown that it increases the concentration of the sulfonic groups in the mesoporous silica relative to post-formation grafting (I. Diaz et al., Stud. Surf. Sci. Catal., 135, 1248 (2001). One approach demonstrated previously involves one-step synthesis based on the simultaneous hydrolysis and condensation of tetraethoxysilane (TEOS) with 3-(mercaptopropyl)trimethoxysilane (MPTMS) in the presence of template surfactant using in situ oxidation of the thiol groups with H2O2. Melero et al. has shown that the acid strength of the sulfonic groups in the mesoporous materials can be adjusted by choice of the organosulfonic precursor (J. A. Melero et al., J. Mater. Chem., 12, 1664 (2002).
For example, mesoporous catalysts containing sulfonic acid groups and, optionally internal methyl groups have been reported to be efficient catalysts in the esterification of glycerol with fatty acids, where high yields of mono-esters are obtained. See, e.g., W. D. Bossaert et al., J. Catal., 182, 156 (1999); I. Diaz et al., Appl. Catal. A., 242, 161 (2003); I. Diaz et al., J. Catal., 193, 295 (2000); D. Magolese et al., Chem. Mater., 12, 2448 (2000). Similar SO3H silicates have been used to tetrahydropyranylate ethanol. M. H. Lin et al., Chem. Mater., 10, 467 (1998). Mesoporous silica functionalized with arenesulfonic acid groups has been used to catalyze the Fries rearrangement of phenyl acetate to 2- and 4-hydroxyacetophenones. J. A. Melero et al., J. Mater. Chem., 12, 1664 (2002).
However, a continuing need exists for a simple method to form (lower)alkyl esters of fatty acids in the environment of triglyceride-containing feedstocks.